Mohs Hardness Scale

The Mohs hardness scale is a qualitative ordinal scale from 1 to 10 that ranks minerals by their scratch resistance. A harder mineral scratches a softer one: Talc (1) is scratched by all minerals, while Diamond (10) scratches everything. The scale is relative, not linear — the absolute hardness gap between Corundum (9, absolute 400) and Diamond (10, absolute 1,500) is much larger than between Talc (1) and Corundum (9). Friedrich Mohs developed this scale in 1812.

The tool provides approximate Mohs-to-Vickers conversions: Mohs 1 equals about 1 HV, Mohs 2 about 3 HV, Mohs 3 about 9 HV, Mohs 5 about 48 HV, Mohs 7 about 100 HV, Mohs 9 about 400 HV, and Mohs 10 about 1,500 HV. Note that the Mohs scale is a relative ranking, not an absolute measurement, so conversions are approximate. For precise industrial measurements, use Vickers, Rockwell, or Brinell hardness testing directly.

The reference includes common field test materials: fingernail (Mohs 2.5) scratches Talc and Gypsum but not Calcite; copper coin (3.0-3.5) scratches Calcite but not Fluorite; glass plate (5.5) is scratched by Orthoclase and above; steel knife or file (5.5-6.5) can scratch up to Orthoclase depending on the steel alloy. For a proper test, scratch the specimen with each reference material, then rub the scratch mark with your finger to distinguish actual grooves from powder residue.

HRC (Rockwell C) uses a diamond cone indenter at 150 kgf for hardened steel and tool steel (range 20-70). HRB (Rockwell B) uses a 1/16-inch steel ball at 100 kgf for mild steel, brass, and aluminum (range 25-100). HV (Vickers) uses a diamond pyramid with variable load (1-120 kgf) and applies to all hardness ranges with a single scale. HB (Brinell) uses a 10mm ball at 3000 kgf for castings, forgings, and large parts (up to HB 650). The conversion formula: HB is approximately 0.95 times HV for HB under 350.

The selection principle is that the abrasive must be harder than the workpiece. The reference covers five abrasives in order of hardness: Aluminum Oxide (Al2O3, Mohs 9, HV 2000) for steel and iron alloys; Silicon Carbide (SiC, Mohs 9-9.5, HV 2500) for non-ferrous metals, ceramics, and glass; Boron Carbide (B4C, Mohs 9.5, HV 3000) for cemented carbide lapping; Cubic Boron Nitride (CBN, Mohs 9.5, HV 4500) for hardened steel and superalloys; Diamond (Mohs 10, HV 10000) for cemented carbide, ceramics, and gemstones.

The reference provides coating hardness data: chrome plating at HV 800-1000 (Mohs about 8.5), electroless nickel at HV 500-700 (Mohs 7-8), nitriding at HV 700-1200 (Mohs 8-9), TiN (titanium nitride) at HV 2000-2500 (Mohs about 9), TiAlN at HV 2500-3200 (Mohs 9+), DLC (Diamond-Like Carbon) at HV 1500-5000, and CVD diamond at HV 8000-10000 (Mohs 10). For cutting tools, TiN, TiAlN, and DLC are common; for molds, CrN and TiN; for wear-resistant parts, DLC and chrome plating.

Human tooth enamel has a hardness of approximately Mohs 5, similar to Apatite (Ca5(PO4)3(F,Cl,OH)), which is fitting since tooth enamel is composed primarily of hydroxyapatite. This means tooth enamel can be scratched by materials of Mohs 6 and above, including Orthoclase feldspar, Quartz (sand particles at Mohs 7), and harder minerals. This is why abrasive particles in certain toothpastes and environmental sand/dust can cause enamel wear over time.

The reference ranks building materials by hardness: drywall (gypsum, Mohs 2) is softest; marble and limestone (Mohs 3-4) are relatively soft and prone to scratching; slate (Mohs 5.5-6); sandstone, granite, and ceramic tile (Mohs 6-7); quartzite (Mohs 7) is among the hardest natural stone options. For flooring durability: marble is less durable than granite, which is less durable than quartzite, which is less durable than engineered quartz. The PEI rating (Porcelain Enamel Institute) should also be considered for tile wear resistance.